This invention relates to telephone systems wherein connections between subscriber stations, or between subscriber stations and lines extending beyond such stations, e.g. office lines, are established by operating a line key which is connected with the called subscirber station or office line, and in which systems the control means for establishing connections are decentralized and arranged mainly at the subscriber stations and at the line transmissions, respectively.
There are two principal kinds of branch systems, i.e. "systems with exchanges" and "series systems." In a "series system" an external office call passes all the series stations connected in parallel to a voice line, and ends in a main series station which is provided with a buzzer. The series branch stations are provided with keys for establishing connections with office subscriber stations. Other series stations than the aforementioned main series station may recieve office calls. Each entitled branch station may likewise receive the call as long as a calling signal is transmitted to the branch stations as, for instance, by means of an additional buzzer. Internal calls among the branch stations are also conducted over the common voice line, each branch station being connected with the others by so-called call lines. The system requires a large number of call lines because all signalling operations are performed by the individual branch stations. Such systems are expensive and complex since they require multi-wire cables. In a modification of the above system the call lines are also used as voice lines, each station of the system being connected to the others by such a line. Hence, in such a system several calls may be conducted simultaneously. Each station may add itself to an established connection so that conference calls may be had.
There are many prior art versions of systems under consideration. In these systems voice information as well as control information is transmitted in conventional, i.e. analog form. As a result, these prior art systems require complex multi-wire cables.
A telephone system which is not subject to all the limitations of the aforementioned prior art systems is disclosed in the paper by Aaronson and Acameora, "A Frequency-Division Multiplex Key Telephone System," IEEE Transactions on Communications Technology, Dec. 1971, pages 1242-1243. The main series station of this system is provided with a space multiplex switching network adapted to connect the voice circuit of each station with each external line and with each internal line. The main series station receives information (receive, hold, connect) from a subscriber station and performs the required control and acoustical and optical signalling functions. A second pair of wires is used for transmitting these control signals between the main series station and all subscriber stations. The independent control of the various functions required at the main series station is effected by a frequency multiplex representation of the various signals. The pair of voice wires is also used to transmit the control signals emanating from a subscriber station to the main series station upon depression of a key at the former.
The principal object of the present invention is to provide small branch telephone systems which combine the advantages of branch systems with exchanges and of series systems or, in other words, to provide a small branch telephone system which does not require complex wiring, nor complex centrally located control means.